A search for diffuse bands in fullerene planetary nebulae: evidence of diffuse circumstellar bands^⋆

¹ Instituto de Astrofísica de Canarias, C/via Láctea s/n, 38205 La Laguna, Spain
e-mail: jdiaz@iac.es; agarcia@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Spain
³ Indian Institute of Astrophysics, Bangalore 560034, India
e-mail: nkrao@iiap.res.in
⁴ Consejo Superior de Investigaciones Científicas, Madrid, Spain
⁵ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁶ Actinium Chemical Research srl, via Casilina 1626/A, 00133 Rome, Italy

Received: 30 July 2014
Accepted: 24 November 2014

Abstract

Large fullerenes and fullerene-based molecules have been proposed as carriers of diffuse interstellar bands (DIBs). The recent detection of the most common fullerenes (C₆₀ and C₇₀) around some planetary nebulae (PNe) now enable us to study the DIBs towards fullerene-rich space environments. We search DIBs in the optical spectra towards three fullerene-containing PNe (Tc 1, M 1-20, and IC 418). Special attention is given to DIBs which are found to be unusually intense towards these fullerene sources. In particular, an unusually strong 4428 Å absorption feature is a common charateristic of fullerene PNe. Similar to Tc 1, the strongest optical bands of neutral C₆₀ are not detected towards IC 418. Our high-quality (S/N > 300) spectra for PN Tc 1, together with its large radial velocity, permit us to search for the presence of diffuse bands of circumstellar origin, which we refer to as diffuse circumstellar bands (DCBs). We report the first tentative detection of two DCBs at 4428 and 5780 Å in the fullerene-rich circumstellar environment around the PN Tc 1. Laboratory and theoretical studies of fullerenes in their multifarious manifestations (carbon onions, fullerene clusters, or even complex species formed by fullerenes and other molecules like PAHs or metals) may help solve the mystery of some of the diffuse band carriers.